Surface Chemical Analysis on the Corrosion of Alloys in the Supercritical Water Oxidation of Halogenated Hydrocarbon

Abstract

A surface chemical analysis on the corrosion of various alloys under supercritical water oxidation (SCWO) conditions with relatively high concentrations of halogenated compounds and hydrogen peroxide was conducted using an Auger electron spectroscopy/scanning Auger-electron spectroscopy. The tested alloys (Inconel 600, Hastelloy C-276, Monel 400, stainless steel (SUS 316), Titanium G2, and Zirconium 702) were exposed to the same conditions: 4000 mg/L of 2,4-DCP at 400 °C and 250 bar, with 700% stoichiometric H2O2 in a Hastelloy C-276 batch reactor. Even under the supercritical water (SCW) condition of 4.8 pH without H2O2, a certain degree of corrosion was observed on the surface of all the alloys, especially SUS 316. Under the severe SCWO condition with excess H2O2, the surface corrosion of all the alloys was significant, but the types of corrosion on the alloy surfaces differed. Chromium in the SCWO process that contained H2O2 for halogenated compounds could potentially lead to the corrosion type and degree of alloys, but a certain amount of nickel depletion was also observed. Among the alloys tested, Titanium G2 was the most resistant to corrosion, under the conditions of the experiment. Considering that the surfaces of the alloys were covered by a carbon-contaminated layer, it may be concluded that the metal oxides or metal ions on the surface of the alloy have a role in forming the carbon-contaminated layer in the decomposition of halogenated compounds under SCWO.

Corrosion Properties of Candidate Materials in Supercritical Water Oxidation Process

“…The 316L alloys showed the highest corrosion rate about 50 mmyr -1 among all alloys, indicating austenitic stainless steel was not suitable for chlorinated feed in SCWO (29). When the test of decomposing organic matter was conducted, it was always found that the carboncontaminated covered the surface of alloys due to the incomplete degradation of organic matter (26,28). It is known that the high temperature subcritical water has higher corrosive and solubility than supercritical water.…”

“…Corrosion of Gr2 became severe and Gr12 showed a low corrosion rate among candidate titanium based alloys in the presence of sulfate or phosphate (67). The titanium Gr2 showed more resistance to corrosion than other alloys (such as Inconel 600, Hastelloy C-276, Monel 400, 316 SS and Zirconium 702) in the SCWO process for decomposing halogenated compounds (26,45,68). The titanium displayed more resistance to corrosion than 304 SS in high temperature subcritical solution with HCl (76).…”

“…Carbon contamination was significant when treating organic wastes with SCWO, implying the destruction reaction occurs also on the material surface. Therefore, the high content of carbon could be observed in the outer layer oxides (26).…”

“…Oxygen ions migrated to the matrix and combined with the chromium to form chromium oxide. In contrast to other metal elements like Fe, Cr, Mo, the Ni usually presents a worst serious depletion and the highest concentration of nickel is detected in the corrosion effluent, especially in acidic chlorinated solution (26,40,46,52,58,66,93,99,100). The pure chromium presents greater corrosion rate than Inconel 625 in supercritical water containing HCl, NaCl and H 2 O 2 (52).…”

“…In recent years, lots of studies about SCWO corrosion focused on the surface chemical and the migration of Table 1. Physicochemical properties of water as a function of temperature and pressure (1 elements, which were helpful to understand the corrosion mechanism (24)(25)(26)(27). The alloy element and element content usually played an important role in the corrosion behaviors of materials under SCWO conditions.…”